Remove SkiaSharedBufferStream as SharedBuffer is not sharable.

SharedBuffer is currently being used for both images and webfonts in Blink.
However, SharedBuffer is not thread safe and is Refcounted
(and not ThreadSafeRefcounted) which means it will assert early on access
from multiple threads. This had not been noticed previously due to our use
of SharedBuffer being (accidentally) thread safe, and Blink in general
being single threaded. However, with recent cross thread picture playback
this is no longer the case and Blink will need to begin using thread safe
data buffers for images and webfonts.

This change is less than ideal as it makes a copy of the data.
The reason for SkiaSharedBufferStream was to avoid this copy.
This change adds getAsSkData to SharedBuffer. This might be modified
in the future when Blink has it's own native thread safe ref counted data
abstraction.

R=fmalita@chromium.org

Review URL: https://codereview.chromium.org/70083005

git-svn-id: svn://svn.chromium.org/blink/trunk@162238 bbb929c8-8fbe-4397-9dbb-9b2b20218538